Sensitive photodetectors, photomultipliers, etc., are susceptible to being blinded, saturated or otherwise being driven into a nonlinear range. During high intensity flashes, data is lost and the sensing devices can and do get damaged.
Perhaps the first type of light intensity control that comes to mind is found in an ordinary camera. To obtain a satisfactory exposure on a photographic film, a variable aperture iris is adjusted to predetermine the amount of light incident on the film to prevent overexposure or underexposure. This well-known capability also is inherently present in the human eye, for in the eye, the iris varies the aperture through which light passes onto the retina and automatically provides protection.
Sophisticated modern optical systems have included a variety of mechanical or solenoid operated shutters to control light intensity. An example of this is disclosed by C. J. Seur in his U.S. Pat. No. 2,868,100 entitled "Camera Comprising a Pick-up Tube and a Lens Objective with a Variable Effective Aperture Ratio". An elastic taut light-previous skin is provided with a means to adjust its tension. As the tension is varied, the permeability of the skin is changed and the quantity of light striking a photocathode of a pick-up tube will not be overexposed. While a noteworthy advance in the state-of-the-art, the mechanical coaction, allegedly automatic, leaves something to be desired when it is considered that rapidly varying flashes of ambient light might be expected.
Similarly, the principal deficiency of many of the other mechanical shutter or iris devices is that they too have slow responses, being on the order of 50 to 100 msec. A typical system sensing electromagnetic energy from a relatively low ambient light background can be blinded, driven nonlinear or irreversibly damaged by sudden light flashes or intense light sources such as flares, flashlamps, pulsed lasers, or the sun. The sophisticated sensing systems, particularly those employed by the military for surveillance, cannot risk being neutralized by overloading bursts of radiation. Thus, there is a continuing need in the state-of-the-art for a device which automatically and rapidly protects photosensing devices from sudden intense bursts of radiation.